Error handling, when tsserver cannot be found

[svenefftinge]

Since we no longer ship typescript with this npm package, there are scenarios in which no tsservercan be found. We need to send a message to the user, telling her what to do.

The corresponding exception https://github.com/theia-ide/typescript-language-server/blob/master/src/lsp-server.ts#L85

[rchl]

With #292 we'll fail the initialize request with an Could not find a valid tsserver version. Exiting. error if we can't resolve typescript.